18mm to 55mm canon infinite focus.

[NIGHTBOY]

Hi guys. Just been out taking some pics of the sky but struggling to find that infinite sweet spot. I'm using a canon 700d, ISO 800, 18mm, f4.0 and the shutter 20/15 seconds. Now stood behind the camera I've tried winding the focus ring anti clock wise to the max will give the best results but its still not spot on.

Any pointers??

Many thanks.

[Leveye]

You could make your own tiny Bahtinov mask. Worked for me.

[NIGHTBOY]

Thanks leveye. Do you find that's its a tiny bit towards clockwise after going all the way anti clockwise (if you follow)

[Leveye]

Every one of those kit lenses is off. Use the mask and be sure.

[ronin]

It does tend to be fully one way then back a bit, simply so that they can get to infinity every time. they in effect add a bit more rotation then is necessary.

Problem being how much back.

Making a mask is one idea but don't they rely on the focal length and possibly aperture being fixed. If a normal zoom I would have therefore expected the mask to work for one setting alone.

[baggywrinkle]

There is extra travel in a camera lens to allow for variations of temperature.

I made some very small marks where I found the best point on a couple of lenses. Tends to show up more on longer focal length lenses.

Masks work, an alternative with the canon is to onvest in a copy of Backyard Eos. Has a great focusing app built in. Plus all the other bells and whistles to set up a program of exposures.

[NIGHTBOY]

Thing is tho in often out with just m camera so I don't want to be carrying a laptop where ever I go.

[frugal]

I have three canon lenses, two of them have the focal length marked on them, one does not. On all of them infinity is not right at the end of the focus range, but a little bit back from the end. Two of the lenses have a little focus length gauge and it clearly focuses at infinity before the far end of the focus range.

[NIGHTBOY]

Hi frugal. Yes my 70/300mm sigma lens has got the infinity mark on it and that's just a little way from the end. The thing with the 18/55mm canon is the focusing ring doesn't stop turning, you could wind it which ever way until the cows come home. Mo marks on it either so slightly annoying. I take it that you can only use this method when the lens it at its minimum focal length??

[frugal]

 The thing with the 18/55mm canon is the focusing ring doesn't stop turning, you could wind it which ever way until the cows come home. 

I have the same problem with the 100-300 lens. I have found that if I turn it to the "stop" as it were, the ring is slightly harder to turn after that. So I wait until the feel changes which indicates that I have reached the end of the focus and then I can wind it back a bit.

I do have to say that the only way I have had any real success focusing is to use Backyard EOS and the live view.

[NIGHTBOY]

Does that require a laptop?

[frugal]

Unfortunately it does.

[NIGHTBOY]

Ahh I see. Well it's something I can do from home I suppose. I normally take my camera and tripod when waking the dog so I wont be carrying a laptop too!!

Could you please explain how backyard eos is better for focusing rather than having the camera connected to a HDTV. Does it remotely control the focus??

[JamesF]

Ahh I see. Well it's something I can do from home I suppose. I normally take my camera and tripod when waking the dog so I wont be carrying a laptop too!!

Could you please explain how backyard eos is better for focusing rather than having the camera connected to a HDTV. Does it remotely control the focus??

Both BYE and APT can capture images from the camera and use measurements from a star in the image to refocus the lens to obtain the theoretically optimal focus.

James

[frugal]

Ahh I see. Well it's something I can do from home I suppose. I normally take my camera and tripod when waking the dog so I wont be carrying a laptop too!!

Could you please explain how backyard eos is better for focusing rather than having the camera connected to a HDTV. Does it remotely control the focus??

It lets you select a star on screen, and then it gives you a digital readout of the size of the star. The better the focus, then smaller the star will appear. The advantage is that you can see this number on screen in a big font whilst you are standing at the camera. As you adjust the focus you can see if the number gets smaller (better focus) or larger (worse focus).

[NIGHTBOY]

So how does it do this, do you still have to manually adjust it? I'd like to know as I can do his then mark my lens.

[NIGHTBOY]

Ahh thanks frugal nicely put. Ill sure be giving it a bash. Does it alter that much from star to star? I generally stay on 18mm and take 20 second exposures

[JamesF]

So how does it do this, do you still have to manually adjust it? I'd like to know as I can do his then mark my lens.

APT claims to be able to do it all for you, though I've never used it.  I was pretty sure BYE would do the same.  There is also frugal's method which I do use if the camera is on a scope rather than having a lens.

James

[NIGHTBOY]

Ok so say my camera is on my scope facing Jupiter with bye on the lappy. I then choose Jupiter on the screen and adjust the focuser until I get the lowest number and then it's perfectly focused??

[JamesF]

Ok so say my camera is on my scope facing Jupiter with bye on the lappy. I then choose Jupiter on the screen and adjust the focuser until I get the lowest number and then it's perfectly focused??

Ah, no, doubtful.  The focusing calculation is done on the basis that the target is a star (and in particular the way light from a distance point source arrives on the sensor).  I'm pretty certain that Jupiter wouldn't work.

James

[Pete Presland]

i find all my lens require the focus to be just short of the end of travel. i just zoom in to max on playback and check the star size against the white square which get smaller the more you zoom in.if at max zoom the star is smaller than the square i know the focus is good (hopefully)

[NIGHTBOY]

So BYE is only for taking pictures of single stars?

[NIGHTBOY]

So BYE is only for taking pictures of single stars?

[JamesF]

So BYE is only for taking pictures of single stars?

No, you can use BYE or APT to take images of anything I imagine.  I use APT for wide field imaging and solar/lunar imaging with a DSLR.  But the focusing scoring is based on modelling how light from a distant point source appears on the camera sensor, so it isn't likely to work for planets.

James

[frugal]

Ok so say my camera is on my scope facing Jupiter with bye on the lappy. I then choose Jupiter on the screen and adjust the focuser until I get the lowest number and then it's perfectly focused??

The problem with focussing on planets is that they are not point light sources, they are a few arc seconds across. This means that you can not focus properly by trying to get the point as small as possible.

If you want to view Jupiter. Do the focussing on a nearby bright star, and then once you have the focus right slew the scope across to Jupiter. As they are both "infinitely far away" the focus should be the same for both the star and the planet.

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